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Subject: Re: bug in tag handling in blk-mq?
To:     Mike Galbraith <efault@gmx.de>,
        Paolo Valente <paolo.valente@linaro.org>
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <1e51eb57-6d9b-a53c-9cd6-2adfc86b21b5@kernel.dk>
Date:   Wed, 9 May 2018 11:01:52 -0600
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On 5/9/18 10:57 AM, Mike Galbraith wrote:
> On Wed, 2018-05-09 at 09:18 -0600, Jens Axboe wrote:
>> On 5/8/18 10:11 PM, Mike Galbraith wrote:
>>> On Tue, 2018-05-08 at 19:09 -0600, Jens Axboe wrote:
>>>>
>>>> Alright, I managed to reproduce it. What I think is happening is that
>>>> BFQ is limiting the inflight case to something less than the wake
>>>> batch for sbitmap, which can lead to stalls. I don't have time to test
>>>> this tonight, but perhaps you can give it a go when you are back at it.
>>>> If not, I'll try tomorrow morning.
>>>>
>>>> If this is the issue, I can turn it into a real patch. This is just to
>>>> confirm that the issue goes away with the below.
>>>
>>> Confirmed.  Impressive high speed bug stomping.
>>
>> Well, that's good news. Can I get you to try this patch?
> 
> Sure thing.  The original hang (minus provocation patch) being
> annoyingly non-deterministic, this will (hopefully) take a while.

You can verify with the provocation patch as well first, if you wish.
Just need to hand-apply since it'll conflict with this patch in
bfq. But it's a trivial resolve.

-- 
Jens Axboe